Field of the Invention
The present invention relates to a control system for an internal combustion engine, for controlling the engine such that a fuel consumption ratio is minimized.
Description of the Related Art
A conventional control system for an internal combustion engine is disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2006-193137. This engine is installed on a hybrid vehicle as a motive power source. The control system has a control map in which are set a large number of operating point data items that define the relationship between the rotational speed of the engine and an engine torque, a reference operating line Q connecting the operating point data items, and nine equal output lines Pi (i=1 to 9) associated with nine engine outputs, respectively.
In this control system, an operating point-learning process shown in FIG. 4 of the mentioned publication is executed during operation of the engine such that a fuel consumption ratio is minimized. In this learning process, in a step A11, an operating point on one of the equal output lines Pi (hereinafter referred to as the “currently set operating point”), associated with an output demanded of the engine, is referred to, and in a step A12, a fuel consumption ratio is calculated based on data of the rotational speed of the engine and an engine torque on the currently set operating point. Further, in a step A13, it is determined whether or not the calculated fuel consumption ratio is minimum. If it is determined that the calculated fuel consumption ratio is not minimum, when the output of an electric motor can cover the engine output, the process proceeds to the step A11 again, wherein the operating point is changed from the currently set operating point to a different point on the equal output line Pi, and in the step A13, it is determined whether or not a fuel consumption ratio associated with the changed operating point is minimum. If it is determined that the calculated fuel consumption ratio is not minimum, in a step A14, it is determined whether or not the output of the electric motor can cover the engine output. If it is determined that the output of the electric motor cannot cover the engine output, the present process is immediately terminated.
On the other hand, when it is determined that the output of the electric motor can cover the engine output, the process returns to the step A11. More specifically, when it is determined that the calculated fuel consumption ratio is not minimum, if it is determined that the output of the electric motor can cover the engine output, the above-described steps A11 to A14 are repeatedly executed until an operating point that minimizes the fuel consumption ratio (hereinafter referred to as an “optimum operating point”) is determined. When the optimum operating point is determined, an optimum operating point stored in a memory is updated to the determined optimum operating point and stored in respective steps A15 and A16, followed by terminating the learning process.
The above-described conventional control system is configured such that the process for searching for an optimum operating point is repeatedly executed until the optimum operating point is determined, and hence, for example, when atmospheric pressure has changed, it takes time to determine an optimum operating point in a manner coping with the change in the atmospheric pressure, and during the time period, it is impossible to operate the engine such that the fuel consumption ratio is minimized. This can result in the degraded fuel economy.